Compounds interactions during simultaneous biodegradation of hydrophobic n-hexane and hydrophilic methanol vapors in one- and two-liquid phase conditions

Abstract

Simultaneous biodegradation of n-hexane (H) and methanol (M) was studied in batch culture in the concentration range of 5−10 g m⁻³ and 1−5 g m⁻³, respectively, in the absence and presence of 5−10 % v/v silicone oil. The addition of oil likely increased the mass transfer of H and ended up to H removal efficiency (RE_H) improvement from 72 –86 % to 92–94 %. In contrast, M removal efficiency (RE_M) decreased from 15-–34 % to 6–24 %, due to an increase in the M partition coefficient from 0.00018 to 0.003. Based on statistical analysis, RE_H and RE_M of 90 % and 24 % were obtained at optimum condition of 5 g m⁻³, 1 g m⁻³, and 5 % v/v for H, M, and oil fraction, respectively. Kinetic study based on Michaelis- Menten for H biodegradation showed that the presence of 5 % v/v oil could decrease the inhibition effect of the presence of M by increasing V_max from 123 to 133 mg _H (g _biomass d)⁻¹ and decreasing K_m from 21 to 17 g m⁻³. Also, degree of mineralization increased from 27–29 % in the absence of oil to 35–43 % in the presence of oil for the identical experiments. According to 16S rDNA sequence analysis, Bacillus cereus and Pandoraea pnomenusa were identified as the main H and M degrading species, respectively, with REs >99 %.

Introduction

Volatile organic compounds (VOCs) like methanol and n-hexane are emitted into the atmosphere by various chemical industries. These emissions can cause major human health problems like blurred vision, dizziness, nausea, and headache (Andrews et al., 1987; United States Environmental Protection Agency, 2005; Amin et al., 2017). Methanol (M) is a hydrophilic compound which is completely miscible with water. In contrast, n-hexane (H) is a hydrophobic compound with limited solubility in the aqueous phase (9.5 mg l⁻¹ at 25 °C and 1 atm) based on its high dimensionless Henry's law constant of 73.7 (Hernández et al., 2010). In some industries such as printing and publishing, polymers and man-made fibers, pulp and paper, and organic chemicals, a mixture of H and M vapors may be produced (Zamir et al., 2015; Zehraoui and Sorial, 2015).

To decrease the VOCs emissions to the environment, physico-chemical techniques (e.g., incineration, adsorption, condensation, etc.) can be used for the removal of VOCs. Besides, biological processes including biofilters (BFs), biotrickling filters (BTFs), stirred bioreactors, etc. are also well-known due to their cost-effectiveness and environmentally friendly properties (Mudliar et al., 2010; Rene et al., 2015; Yang et al., 2019). In gas-phase bioreactors, the gaseous pollutant (e.g., H and M) is transferred from gas to the liquid and suspended microbial or biofilm phase, where the biodegradation takes place. Therefore, there are two sorts of limitations regarding the removal of pollutants in the associated bioreactors that are mass transfer and kinetic limitations (Ferdowsi et al., 2017). Mass transfer limitation from gas to liquid phase is usually a concern for hydrophobic VOCs like H. In other words, the elimination of H vapor in bioreactors is intrinsically limited by low bioavailability of the pollutant in water phase due to poor mass transfer from gas to aqueous phase (San-Valero et al., 2017). On the contrary, hydrophilic compounds like M are readily available to the microbial phase in water (Mohseni and Allen, 2000). Thus, M biodegradation could be significantly affected by increasing the risk of toxicity for microorganisms due to the M accumulation beyond a critical level in the water/biofilm phase, resulting to a substrate inhibition in the biofilm (kinetic limitation) (Ferdowsi et al., 2017). Synergistic or antagonistic interactions are commonly observed during the simultaneous biodegradation of multiple VOCs. These interactions may either enhance or inhibit biodegradation of each individual compound. Synergistic substrate interactions include co-metabolism and increased biomass growth, whereas antagonistic interactions include toxicity at high concentrations and competitive inhibition (Hammershøj et al., 2019). The extent of the effects of multiple substrates on biodegradation depends on the mixture composition, the initial concentrations, and the biomass composition, in particular, whether a pure or mixed bacterial culture is used (Xu et al., 2020; Yang et al., 2018). For example, the mutual interactions between substrates were studied by Zehraoui et al. (2012) during the elimination of H/M mixture in a one-liquid phase BTF (OLP-BTF). The presence of H (0.03−0.5 g m⁻³) did not significantly affect the almost complete removal efficiency (RE) of M (RE_M > 98 %), whereas RE of H (RE_H) in the presence of M was enhanced from 88 to 96 % at H inlet concentrations < 0.1 g m⁻³ (Zehraoui et al., 2012). Also, it was reported that the presence of 4-methyl-2-pentanone as a hydrophilic compound could enhance the biomass growth and degradation of H at low 4-methyl-2-pentanone/H mixing ratio of 1:6 g g^–1. At higher mixing ratios of 1:2 or 1:1 g g^–1, RE_H decreased due to competitive inhibition between 4-methyl-2-pentanone and H (Cheng et al., 2020).

In order to overcome mass transfer limitation and low solubility of hydrophobic VOCs like H in gas-phase bioreactors, addition of an organic phase like silicone oil to the water phase has been suggested (Wu et al., 2018). This non-aqueous phase (NAP) should be non-biodegradable and immiscible with water and should have a high affinity for the absorption of target hydrophobic compounds from the gas phase (Angelucci et al., 2019; Guillerm et al., 2017; Wu et al., 2017). In this regard, NAP acts as an intermediate phase between gas and microbial phases, increasing the bioavailability of the pollutant for biocatalysts by absorbing it from gas phase (Angelucci et al., 2019; Boojari et al., 2019). Arriaga et al. (2006) reported that maximum RE_H at initial concentration of 3 g m⁻³ in a one-liquid phase (OLP) stirred reactor was only 28 % at gas residence time of 1 min, while RE_H reached 67 % in the presence of 10 % v/v silicone oil as an NAP in a two-liquid phase (TLP) reactor under the same operating condition. Fazaelipoor et al. (2006) obtained REs of 66 % and 97 % for H removal in the absence and presence of 25 % v/v silicone oil, respectively in two identical BTFs. Furthermore, it is hypothesized that the addition of NAP can also decrease the risk of M toxicity on the H-degrading species. In this case, H-degrading species can develop from aqueous phase into NAP to access directly to the accumulated H in NAP (Hernández et al., 2012; Muñoz et al., 2013). Therefore, H-degrading microorganisms would be much safer in NAP phase in terms of M toxicity effects since M is more available in the aqueous phase than NAP. Furthermore, addition of silicone oil may decrease the interaction between pollutants in a mixture and improve their biodegradation. When a mixture of hydrophobic (e.g., H) and hydrophilic (e.g., M) compounds is present in bioreactors, pollutants may interact with each other. In other words, one may enhance or decrease the removal of another compound (Mohseni and Allen, 2000). These interactions mainly change the kinetics of biodegradation by changing the metabolic pathways for the degradation of each compound (Cheng et al., 2020; Mohseni and Allen, 2000; Rene et al., 2015). Addition of silicone oil could split up the H molecules from the mixture and transfer them to the NAP to be available to the hydrophobic biomass inside the NAP, while M is available to the related degraders in the aqueous phase.

Most studies have focused so far on the effect of NAP in a continuous process like two-liquid phase bioreactors. In a continuous biodegradation, different phenomena like adsorption on packing materials or fluid hydrodynamics in bioreactors as well as some operating parameters like gas flow rate and retention time may influence the bioreactor performance. Therefore, the effect of NAP on VOC biodegradation could not be clearly observed with the interference of any external transport phenomena such as convective mass and momentum transfers and axial dispersion. In addition, when multiple pollutants are continuously fed to the bioreactors, the mutual effects of pollutants as well as pollutants/NAP may not be precisely examined. In batch tests, the effects of NAP on the solubility of each compound and on the kinetics of biodegradation can be clearly observed. To our best knowledge, no study has focused on the mentioned effects in batch biodegradation of a hydrophobic and a hydrophilic pollutant with and without an NAP.

The aim of this study was to investigate the simultaneous biodegradation of M as a hydrophilic and H as a hydrophobic pollutant in the absence and presence of silicone oil as the NAP. The biodegradation experiments were carried out in batch scale to study the probable effects and interactions between H, M, and silicone oil concentrations in the culture.